The ideal cancer drug should target a non-redundant fraction continuously necessary for tumor maintenance, but dispensable for maintenance and function of my normal tissues. Hence, the most common logic is to target gene products that are specifically mutated in cancer, on the basis that these mutant molecules would be the likely “drivers” of the cancer and, perhaps, less critical for normal tissues. For these reasons, much attention has focused on cataloguing recurring lesions in specific cancer types. Unfortunately, there are several problems to this approach. First, most solid human cancers pass through episodes of genomic instability and exhibit a mutational noise that can obscure the “driver” mutations and their attendant effector pathways. Second, cancers are the end result of a process that involves transitions through multiple evolutionary bottlenecks. Each bottleneck may require a specific type of mutation whose function is thereafter dispensable for tumor maintenance and, consequently, not a good therapeutic target after that point in the tumor's evolution.
Myc is a basic helix-loop-helix leucine zipper (b-HLH-LZ) protein involved in growth control and cancer, which operates in a network with the structurally related proteins Max, Mad and Mnt. Myc/Max dimers activate gene transcription and induce cell proliferation or apoptosis, Mad/Max and Mnt/Max complexes act as repressors and cause cell growth arrest and differentiation. All dimers recognize the same DNA consensus site, the CACGTG E-box.
Myc is tightly regulated in normal cells, where its levels are higher in proliferating and lower in non-proliferating. Aberrantly high and/or deregulated Myc activity is causally implicated in most cancers and often associated with aggressive, poorly differentiated and angiogenic tumors. The deregulation of Myc expression is due to overexpression through gene amplifications, loss of transcriptional control, impaired degradation or increased stabilization. This results in aberrant proliferation, increased survival, changes in metabolism, angiogenesis and inflammation, all of which represent major hallmarks of cancer. Multiple studies substantiated the crucial role of Myc in governing intracellular and extracellular aspects of tumorigenesis suggesting that targeting its function would be therapeutically valuable.
It is known that down-regulation of myc by a BET bromodomain inhibitor results in the regression of multiple tumor types (Delmore, J. E., et al., 2011, Cell, 146: 904-917). While this approach displays good potential, it presents some limitations such as toxicity and numerous off targets effects.
Many small molecules disrupting the Myc/Max interaction have displayed low specificity in cellulo (Prochownik, E, V. and Vogt, P. K., 2010, Genes Cancer 1, 650-659).
A Myc inhibitor, however, has yet so become clinically available and its design presents various caveats: first Myc is a nuclear transcription factor, which is consequently more difficult to reach than membrane or cytoplasmic molecules: second, Myc does not have an enzymatic “active site” that could be targeted; third, the Myc family comprises 3 different proteins, c-, N and L-Myc, which in certain conditions are functionally redundant, so all of them require simultaneous inhibition. Furthermore, there have been concerns that Myc inhibition would induce serious side effects by inhibiting proliferation of normal tissues. For all these reasons, making a Myc inhibitor drug is challenging.
Omomyc is a dominant-negative MYC mutant comprising the b-HLH-LZ domain of Myc and harboring four amino acid substitutions in the leucine supper of Myc (Soucek, L. et al., 1998, Oncogene 17, 2463-2472; Soucek, L, et al. (2002), Cancer Res 62: 3507-3510). The amino acid substitutions E61T, E68I, R74Q, and R75N confer altered dimerization specificity to the protein, which retains the ability to bind its natural partner Max and to form homodimers and heterodimers with wild type c-, N- and L-Myc.
Because of these properties, Omomyc is able to prevent Myc-dependent gene transactivation functions both in vitro and in viva by negating the ability of Myc to bind its DNA recognition binding site, the E box (Savino, M. et al., 2011, PLoS One 6, e22284; Soucek, L. et al. (2004), Cell Death Differ II, 1038 1045). At the same time, Omomyc strongly potentiates Myc-induced apoptosis in a manner dependent on Myc expression level and thereby strengthens Myc transrepression activity. Omomyc thus prevents Myc binding to promoter E-boxes and transactivation of target genes while retaining Miz-1-dependent binding to promoters and transrepression. In the presence of Omomyc, the Myc interactome is channeled to repression and its activity switches from a pro-oncogenic to a tumor-suppressive one.
TRE-Omomyc:CMVrtTA mice, in which Omomyc expression is controlled by a tetracycline-responsive promoter element and the widely expressed rtTA transactivator is driven by a CMV promoter, exhibit high Omomyc expression in most tissues following administration of doxycycline (Soucek et al., 2008, Nature, 455; 679-683). These mice were crossed with the well-established LSL-KrasG12D marine model of lung tumorigenesis. Just 3 days of Omomyc expression were sufficient to cause dramatic shrinkage of the tumors and one week renders the animals essentially tumor free. Importantly, although other dividing tissues, such as skin, testis and intestine, exhibited significantly decreased proliferation rates during the treatment, and displayed a certain degree of atrophy, the mice exhibited no obvious signs of distress or disease. Moreover, the side effects of Myc inhibition resulting from Omomyc expression are completely reversible and disappear-upon discontinuation of the treatment.
To date, despite the fact that the expression of Omomyc has proven to be an efficacious Myc inhibiting strategy in vivo, it has been applied solely using a gene therapy approach. Indeed, Omomyc is a peptide considered too bulky and unfit for delivery to the desired cellular compartment (Montague M. et al, PLoS One. 2012; 7:e32172. doi: 10.1371/journal.pone.0032172), Savino M. et al., PLoS One. 2011; 6:e22284. doi: 10.1371/journal.pone.0022284) and Genes Dev., 2011, 25: 895-7. doi: 10.1101/gad.2053311.)
Moreover, Omomyc is predicted to display poor ability to cross physiological barriers because of its intrinsic physico-chemical properties (e.g. hydrophobicity, as predicted using Kyte & Doolittle hydropathy plot, Kyte J., Doolittle R. F. (1982) J. Mol. Biol. 157:105-132). In addition, despite the presence of several arginine residues within the basic region of Omomyc, the most recent algorithms predicting spontaneous cell-penetration capacity of peptides do not predict Omomyc to possess such property (Gautam et al. Journal of Translational Medicine 2013, 11:74).
Therefore, providing therapeutic approaches for the treatment of cancer based on b-HLH-LZ domains capable of transducing across the cellular membrane of eukaryotic cells and inhibiting Myc-dependent gene transactivation would be advantageous.